The field of the invention is tumor suppressor genes.
Cancer at the cellular level is characterized by the disruption of multiple regulatory mechanisms resulting from multiple genetic changes. The search for specific genes with major cancer-related effects has focussed on two fundamental processes: control of proliferation, and control of invasion and metastatic spread. Both processes are complex, and the relevant cancer-related changes in gene expression involve both increases and decreases in the activity of particular proteins.
Metastatic spread occurs when primary tumor cells invade into lymphatics and blood vessels, and disseminate to distant organs (Fidler et al., J. Natl. Cancer Inst. 82:166, 1990; Liotta et al., Cancer Res. 51:5054, 1991; Nicolson, Semin. Cancer Biol. 2:143, 1991; and Chen, Current Opin. Cell Biol. 4:802, 1992). The multiple steps involved in metastasis include proteolytic attack on the basement membrane and extracellular matrix (ECM), adhesion to endothelial cells leading to intravasation, and later extravasation from the circulatory system into tissues such as lung and bone in which the tumor cells are able to proliferate. In normal cells, these processes of invasion and metastasis are blocked by an intricate array of genetically programmed regulatory mechanisms. Overcoming these protective barriers to invasion and metastasis requires multiple changes in gene expression, resulting in gain or loss of gene functions that contribute to tumor progression.
Increased proteolytic activity augments invasion, as evidenced by the increased activity of serine proteases (Testa et al., Cancer Metastasis Rev. 9:353, 1990; Dano et al., Adv. Cancer Res. 44:139, 1985; Foekens et al., Cancer Res. 52:6101, 1992; Ossowsky, Cancer Res. 52:6754, 1992; Sumiyoshi, Int. J. Cancer 50:345, 1992; Duffy et al., Cancer Res. 50:6827, 1992; and Meissauer et al., Exp. Cell Res. 192:453, 1991, metalloproteases (Birkedal-Hansen (ed) Proceedings of the Matrix Metalloporteinase Conference, Destin, Florida, Gustav Fischer Verlag, 1990; Matrisian et al., Am. J. Med. Sci. 302:157, 1991; Stetler-Stevenson, Cancer Metastasis Rev. 9:289, 1990; DeClerck et al., Cancer Res. 52:701, 1992; Bassett, Nature 348:699, 1990; Wolf et al., Proc. Natl. Acad. Sci. USA 90:1843, 1993; and Sato et al., Oncogene 7:77, 1992), and cathepsins (Rochefort et al., Cancer Metast. Rev. 9:321, 1990; and Kobayashi et al., Cancer Res. 52:3610, 1992) in invasive tumor cells. The principal serine proteases known to be associated with tumor invasion mediate the plasminogen activation cascade. In this pathway, plasminogen is converted by plasminogen activators to plasmin, which is a wide-spectrum serine protease that degrades many components of the ECM directly, or indirectly via the activation of metalloproteases. The activity of the plasminogen activators is negatively regulated by plasminogen activator inhibitory proteins: PAI-1, PAI-2, and protease nexins (Chen, Current Opin. Cell Biol. 4:802, 1992).
The elevated expression of uPA (urokinase plasminogen activator) in breast carcinomas and other cancers has been reported by numerous investigators since the 1970's (Testa et al., Cancer Metastasis Rev. 9:353, 1990; Dano et al., Adv. Cancer Res. 44:139, 1985; Foekens et al., Cancer Res. 52:6101, 1992; Ossowsky, Cancer Res. 52:6754, 1992; Sumiyoshi, Int. J. Cancer 50:345, 1992; Duffy et al., Cancer Res. 50:6827, 1992; and Meissauer et al., Exp. Cell Res. 192:453, 1991; Heidtmann et al., Cancer Res. 49:6960, 1989; Sumiyoshi et al., Thromb Res. 63:59, 1991; Reilly et al., Int. J. Cancer 50:208, 1992, Cajot et al., Proc. Natl. Acad. Sci. USA 87:6939, 1990; Foucre et al., Br. J. Cancer 64:926, 1991; Shirasuna et al., Cancer Res. 53:147, 1993; and Janicke et al., Br. Can. Res. & Treat. 24:195, 1993). More recently, it has been shown that PAI-1 and PAI-2 are also elevated in malignancy (Sumiyoshi, Int. J. Cancer 50:345, 1992; Heidtmann et al., Cancer Res. 49:6960, 1989; Sumiyoshi et al., Thromb Res. 63:59, 1991; Reilly et al., Int. J. Cancer 50:208, 1992, Cajot et al., Proc. Natl. Acad. Sci. USA 87:6939, 1990; Foucre et al., Br. J. Cancer 64:926, 1991; Shirasuna et al., Cancer Res. 53:147, 1993; and Janicke et al., Br. Can. Res. & Treat. 24:195, 1993). These findings are inconsistent with the simple paradigm of protease/antiprotease balance in normal cells and its imbalance in tumor cells, thus confusing the issue of how effective uPA may be in metastatic dissemination. Recent studies of the uPA receptor and its importance in modulating uPA activity (Testa et al., Cancer Metastasis Rev. 9:353, 1990; Vassalli et al., J. Cell Biol. 100:86, 1985; and Lund et al., EMBO J. 10:3399, 1991) have indicated further levels of regulation. Thus, although it has been clearly established that uPA is capable of degrading components of the basement membrane and ECM, and that it is often elevated in advanced breast cancer, its precise role in breast cancer invasion remains to be established. Similarly, the importance of PAI-1 and PAI-2 in inhibiting breast cancer invasion is not clearly established (Testa et al., Cancer Metastasis Rev. 9:353, 1990).
The matrix metalloproteases (MMPs) include collagenases and stromelysins. The type IV collagenases (gelatinases), in particular the 72 kDa form, are active in tumor invasion, as indicated by elevated levels in aggressive human tumors (Stetler-Stevenson, Cancer Metastasis Rev. 9:289, 1990). The tissue inhibitors of metalloproteinase activity, TIMP-1 and TIMP-2, target the type IV collagenases, with TIMP-2 interacting exclusively with the 72 kDa form (Stetler-Stevenson et al., Annu. Rev. Cell Biol. 9:541, 1993). Stromelysins-1 (transin) and -2 have been associated with tumor progression in rodent systems, whereas a smaller molecule called PUMP has been identified in human tumor cells (Matrisian et al., Am. J. Med. Sci. 302:157, 1991). Extensive studies of stromelysin-3 have shown a strong correlation with advanced breast cancer (Bassett, Nature 348:699, 1990; Wolf et al., Proc. Natl. Acad. Sci. USA 90:1843, 1993). This protease is secreted by stromal fibroblasts that are proximal to invasive primary breast carcinomas, and not by the epithelial tumor cells, showing the importance of cell-cell interactions in tumorigenic mechanisms.